Device and method for determining the state of a particle filter

ABSTRACT

A device for determining the state, in particular the permeability, of a particle filter has an acoustic source for transmitting an acoustic signal toward the particle filter and has an acoustic receiver for receiving the acoustic signal that has been changed by the particle filter. In addition, a control and evaluation unit is provided for controlling the acoustic source and evaluating the received acoustic signal.

The present invention relates to a device for determining the state of aparticle filter with the characteristics mentioned in the preamble toclaim 1 and a method for determining the state of a particle filter withthe characteristics mentioned in the preamble to claim 9.

PRIOR ART

Particularly in diesel engines, it is difficult to manage theinterrelationship among the production of nitrogen oxides, theproduction of particles, and the consumption of fuel because measurestaken for nitrogen oxide reduction result in increased particulateemissions and fuel consumption. Since particulate emissions cannot beentirely eliminated, suitable measures must be taken.

Particle filters (DPF) in motor vehicles (KFZ) make an importantcontribution to compliance with stricter exhaust limit values. Thesefilters are highly efficient at removing particles from the exhaustflow.

Particles and diesel particles in particular are partially made up ofagglomerations of primary particles composed of hydrocarbon compounds.They are also partly composed of pure carbon. They result fromincomplete combustion in the combustion chamber and range from 50 to 200nm in size. They are usually geometrically irregular in shape. Theparticle quantity and composition depend heavily on engine loadconditions, the injection system, and the chemical composition of thefuel.

Deposited solids reduce the permeability of the filter, as a result ofwhich the exhaust backpressure increases, which in turn leads to engineoutput losses and to increased fuel consumption. For this reason, thefilter is regenerated after it reaches a certain saturation level. Inthis connection, a temperature increase serves to convert the majorityof the oxidizable particulate load into gaseous carbon dioxide CO2 andcarbon monoxide CO and consequently removes it from the filter. Aftersuccessful regeneration, the exhaust backpressure once again assumes asignificantly lower initial value.

The mass of separated particles in the filter cannot be determineddirectly in the motor vehicle itself. Instead, alternative methods havebeen developed to determine the optimum time to initiate regenerationmeasures.

One known method is to use a differential pressure sensor, which detectsthe pressure decrease via the filter body. The differential pressure andtherefore the pressure decrease, however, are determined by both thefilter permeability and the volumetric flow at the filter. Thevolumetric flow in turn depends on the operating conditions of theengine and the exhaust temperature at the filter body, thus resulting ina large degree of cross sensitivity to various influence parameters.With the usual use of ceramic wall flow filters with parallel channels,the flow resistance is the result of the parallel connection of all ofthe filter channels and the surface elements contained in them. If asufficiently large number of these surface elements is cleaned by theregeneration, then the flow resistance decreases so far that a controlunit, which monitors the regeneration, could erroneously conclude thatthe regeneration is complete, and therefore terminate thetemperature-increase measures and thus the particulate oxidation. Thiscan result in undesirable accumulations of combustible material in someregions of the DPF.

Another possibility for determining the load state of the DPF is to usecharacteristic field data to calculate the particulate mass emitted bythe motor through chronological integration over the operating pointsthrough which the engine has passed. This method has a large degree ofuncertainty, particularly in dynamic driving conditions, due to the riskof an error-induced change in particulate emissions. It can be takeninto account for plausibility testing of other measurement methods.

In order to avoid this uncertainty in the calculated particulateemission, another possibility is to measure the particulateconcentration upstream of the DPF with the aid of a sensor.

Due to considerable technical difficulties in several areas, none ofthese basic concepts has yet reached market maturity.

As mentioned above, the sensor is only an aid for estimating the fillstate of the DPF through integration of the particulate quantityproduced by the motor. The regeneration method itself introduces anuncertainty into the determination of the fill state. Since the increasein temperature represents a significant intervention into the motormanagement, this measure should be executed as seldom as possible andfor as short a time as possible. As a result of this, in actual use, theproblem of the DPF not being completely regenerated can arise andregions of the filter can remain partially loaded with residue. Ifadditional particulate matter continues to accumulate during thesubsequent loading phase, then a critical load concentration can occurin certain areas, which leads to an overheating in these areas insubsequent regeneration cycles. This can result in irreversible damageto filter structures.

ADVANTAGES OF THE INVENTION

The device and method according to the present invention have theadvantage that even during the regeneration phase, it is possible toreliably determine the state of the particle filter and therefore, inaddition to the fill state and permeability, its degree of regenerationas well.

The device according to the present invention for determining the stateof a particle filter has an acoustic source for transmitting an acousticsignal toward the particle filter and an acoustic receiver for receivingthe acoustic signal that has been changed by the particle filter. Inaddition, a control and evaluation unit is provided for evaluating thereceived acoustic signal.

The method according to the present invention for determining the stateof a particle filter includes the following steps. An acoustic sourcesends an acoustic signal toward the particle filter. An acousticreceiver receives the acoustic signal that has been changed by theparticle filter and an evaluation unit determines the state of theparticle filter based on this received signal.

Other advantageous embodiments of the present invention ensue from thecharacteristics disclosed in the dependent claims.

The acoustic source of the device according to the present invention canbe a motor, a whistle, or a speaker. If the engine of the motor vehicleis used as the acoustic source, then this reduces the number ofcomponents to be integrated into the exhaust train.

In another embodiment form of the present invention, the acoustic sourceis embodied so that the acoustic signal produced lies in the ultrasonicrange. This makes it possible to improve the spatial resolution.

The acoustic receiver of the device according to the present inventionis advantageously embodied as a microphone.

In a modification of the present invention, the acoustic source isdisposed on one side of the particle filter and the acoustic receiver isdisposed on the other side of the particle filter.

It is also possible for the acoustic source and the acoustic receiver tobe disposed on one side of the particle filter. This makes it possibleto evaluate the altered sound waves that are reflected by the particlefilter.

The device according to the present invention can also be provided withan additional acoustic receiver so that one acoustic receiver isdisposed on one side of the particle filter and the other acousticreceiver is disposed on the other side of the particle filter.

In another embodiment form of the device according to the presentinvention, the evaluation unit is designed so that it can evaluate theamplitudes and/or compare the phase positions of the two acousticsignals to each other.

A modification of the method according to the present invention includesevaluation of the phase and/or the amplitude of the received acousticsignal.

The acoustic source advantageously produces a sinusoidal or pulse-shapedacoustic signal.

It is also advantageous to determine the ambient temperature based onthe acoustic signals sent and received. Knowledge of the ambienttemperature and therefore of the exhaust temperature is helpful forcontrolling the regeneration of the particle filter.

DRAWINGS

A number of exemplary embodiments of the present invention will beexplained in greater detail below in conjunction with three figures.

FIG. 1 is a schematic, cross-sectional depiction of a first embodimentform of the present invention.

FIG. 2 shows the curve over time of a transmitted signal and thereceived signal.

FIG. 3 is a schematic, cross-sectional depiction of a second embodimentform of the present invention.

The present invention makes use of the effect that porous materialsimpede sound as a function of the material's structure. A loaded filterthat has an additional amount of filtrate on the surface, for example,consequently has a different acoustic impedance than an unloaded filter.The measurement of the exhaust backpressure represents the borderlinecase for direct current impedance with an infinitesimal frequency.

Therefore, the permeability of the sound and possibly a change in thephase are initially determined as a function of the frequency forvarious DPF loads. During operation, the acoustic impedance isdetermined and compared to the previously established impedances. Thisdetermines the load state of the DPF. According to one regenerationstrategy, a suitable regeneration procedure can be initiated, forexample, when a certain fill state has been exceeded. The acousticimpedance can be determined in the vehicle in a number of ways.

Description of the First Exemplary Embodiment

In the embodiment form of the present invention depicted in FIG. 1, anacoustic source 1, also referred to as acoustic generator, is disposedin an exhaust pipe 10 upstream of the DPF 3 in the flow direction 10 ofthe exhaust and an acoustic receiver 2 is disposed downstream of the DPF3.

The acoustic source 1 produces a sound wave 1.1 that is then modified bythe particle filter 3 and travels to the acoustic receiver 2, whichreceives it in the form of the modified sound wave 1.2. The receivedsound wave 1.2, which the acoustic receiver 2 has received and convertedinto an electrical signal 1.22, is then evaluated by an evaluation unit4. The evaluation will be explained in greater detail further below.

The two elements comprising the acoustic source 1 and the acousticreceiver 2 can also be installed in the exhaust pipe 10 in the reverseorder.

The acoustic source 1 can, for example, be a piezoelectric speaker or awhistle. The acoustic receiver 2 can, for example, be a microphone.

The sound wave 1.1 travels through the filter structure, which reducesits intensity and shifts its phase. FIG. 2 shows the correspondingsignal curves. The amplitude is plotted on the y-axis and time isplotted on the x-axis.

Knowledge of the excitation signal 1.11 and of the incoming signal 1.22makes it possible to determine both the phase shift and the intensityreduction.

For example, a sinusoidal signal can be used for the excitation signal1.11, thus significantly facilitating the frequency-resolved evaluation.Other signal shapes are also possible and can be broken down into thefrequency components by means of Fourier analysis. It is also possibleto directly evaluate the signal transmission, for example of a shortacoustic pulse. The choice of signal shapes and the measurement of thefundamental signal without an active acoustic generator 1 can be used todetermine the background signal that is generated, for example, by theengine, so that it can be taken into account in the evaluation.

By evaluating the impedance at different frequencies, it is possible toget a picture of the spatial distribution of particle deposits in theDPF 3. It is therefore fundamentally possible to distinguish betweendeposits disposed at the upstream or downstream end of the DPF 3.

The achievable spatial distribution is determined essentially by meansof the wavelength and frequency used. To achieve a local resolution onthe order of magnitude of the particle filter, frequencies of greaterthan 500 Hz are used or better still, frequencies of greater than 2 kHz.If only the overall permeability of the particle filter 3 is ofinterest, is also possible to use lower frequencies.

For temporary pulses, a maximum pulse length therefore lies in the msrange.

The resolution can be significantly improved by using ultrasound.

In general, the radial distribution of particles over the parallelchannels 12 of the particle filter 3 is homogeneous. More detailedinformation about a potential inhomogeneous distribution in the radialdirection can be obtained through the use of several acoustic generatorsand/or acoustic receivers. This allows for a channel-by-channel analysisof the DPF load if necessary.

Description of the Second Exemplary Embodiment

In lieu of the above-described placement of the acoustic source 1 andacoustic receiver 2 on two different sides of the DPF 3 for transmissionof the sound, the placement of the acoustic source 1 and acousticreceiver 2 on one side of the DPF can also generate values for thereflection of the signal. FIG. 3 shows the corresponding placement. Init, the acoustic generator 1 and the acoustic receiver 2 are installedin the exhaust pipe 10 on the upstream side of the particle filter 3 interms of the flow direction 11 of the exhaust. This placement inconnection with pulsed signals corresponds to the operation of a sonardevice.

The signal evaluation occurs in a fashion analogous to those in theabove-described methods.

Description of the Third Exemplary Embodiment

Instead of using a speaker or a whistle as the acoustic source 1,another embodiment form, not shown in the drawings, uses the acousticemission of an already existing source, in particular that of theengine. In this case, two acoustic receivers are advantageously used,one of which is disposed upstream of the DPF and the other of which isdisposed downstream of it. The correlation between the incoming andoutgoing acoustic signal makes it possible to in turn determine theacoustic transmission. The Fourier analysis yields thefrequency-resolved impedance, which is particularly advantageous for theevaluation of the load state.

If the engine noise is plotted in a time-resolved way and the valuesdetected before and after the DPF are correlated with one another, thenin addition to determining the damping, it is also possible to determinethe signal travel time. The signal travel time can be analogouslydetermined through pulse-like acoustic excitation with the aid of anacoustic generator and an acoustic receiver.

If the spatial positions of the two acoustic receivers are known, thenthis makes it possible to determine the speed of sound based on thesignal travel time. The same is true if the spatial positions of theacoustic generator and the acoustic receiver are known. The speed ofsound changes with the root of the absolute gas temperature, which is ofinterest, for example, for controlling the DPF regeneration. The shareduse of the components according to the present invention makes itpossible to eliminate a temperature sensor and test the functionality ofother components, e.g. a preceding oxidizing converter provided toincrease the temperature.

Description of a Fourth Exemplary Embodiment

In another embodiment form not shown in the drawings, a sliding devicemoves an acoustic generator and/or an acoustic receiver into asuccession of various positions so as to achieve a higher radialposition resolution.

With the present invention, the internal state of the DPF is determineddirectly. The cross sensitivity to the volumetric flow that occursduring the differential pressure measurement is significantly reduced.The method and device permit inexpensive components to be used. Thepresent invention can also be used to detect filter defects.

The acoustic components for testing the DPF state can alsosimultaneously perform other monitoring functions for the exhaust train.The recorded spectrum of engine noise as a function of the operatingstate can be used to detect a defect in the exhaust train, e.g. a leak,or a defect in the engine. This is useful as a backup diagnosticprocedure.

1-12. (canceled)
 13. A device for determining the state of a particlefilter, comprising an acoustic source (1) for transmitting an acousticsignal (1.1) toward the particle filter (3), an acoustic receiver (2)for receiving the acoustic signal (1.2) that has been changed by theparticle filter (3), and an evaluation unit (4) connected to theacoustic receiver (2) for evaluating the received acoustic signal (1.2).14. The device according to claim 13, wherein the acoustic source (1) isan engine, a whistle, or a speaker.
 15. The device according to claim13, wherein the acoustic source (1) is embodied so that the producibleacoustic signal (1.1) lies in the ultrasonic range.
 16. The deviceaccording to claim 14, wherein the acoustic source (1) is embodied sothat the producible acoustic signal (1.1) lies in the ultrasonic range.17. The device according to claim 13, wherein the acoustic receiver (2)is a microphone.
 18. The device according to claim 14, wherein theacoustic receiver (2) is a microphone.
 19. The device according to claim15, wherein the acoustic receiver (2) is a microphone.
 20. The deviceaccording to claim 13, wherein the acoustic source (1) is disposed onone side of the particle filter (3) and the acoustic receiver (2) isdisposed on the other side of the particle filter (3).
 21. The deviceaccording to claim 14, wherein the acoustic source (1) is disposed onone side of the particle filter (3) and the acoustic receiver (2) isdisposed on the other side of the particle filter (3).
 22. The deviceaccording to claim 15, wherein the acoustic source (1) is disposed onone side of the particle filter (3) and the acoustic receiver (2) isdisposed on the other side of the particle filter (3).
 23. The deviceaccording to claim 13, wherein the acoustic source (1) and the acousticreceiver (2) are disposed on the same side of the particle filter (3).24. The device according to claim 14, wherein the acoustic source (1)and the acoustic receiver (2) are disposed on the same side of theparticle filter (3).
 25. The device according to claim 15, wherein theacoustic source (1) and the acoustic receiver (2) are disposed on thesame side of the particle filter (3).
 26. The device according to claim20, wherein the acoustic source (1) and the acoustic receiver (2) aredisposed on the same side of the particle filter (3).
 27. The deviceaccording to claim 13, further comprising an additional acousticreceiver (2), with one acoustic receiver (2) being disposed on one sideof the particle filter (3) and the other acoustic receiver beingdisposed on the other side of the particle filter (3).
 28. The deviceaccording to claim 13, wherein the evaluation unit (4) is embodied sothat it is able to evaluate the amplitude of the acoustic signal (1.2)and/or can compare the phase positions of the two acoustic signals (1.1,1.2) to each other.
 29. A method for determining the state of a particlefilter, comprising the steps of utilizing an acoustic source (1) totransmit an acoustic signal (1.1) toward the particle filter (3),utilizing an acoustic receiver (2) to receive the acoustic signal (1.2)that has been transmitted to and changed by the particle filter (3), andutilizing an evaluation unit (4) to determine the state of the particlefilter (3) based on this received signal.
 30. The method according toclaim 29, further comprising the steps of evaluating the phase and/orthe amplitude of the received acoustic signal (1.2).
 31. The methodaccording to claim 29, wherein the acoustic source (1) produces asinusoidal or pulse-shaped acoustic signal.
 32. The method according toclaim 29, wherein the transmitted and received acoustic signals (1.1,1.2) are used to determine the ambient temperature.